NOAA/WDS Paleoclimatology - Seawater Sr/Ca in Florida and Around the World between 2016-2020 CE

Coral skeletal Sr/Ca ratios are a commonly used paleotemperature proxy that relies on stable seawater Sr/Ca ratios (Rsw). Recent observations call this assumption into question at some locations, particularly in the Florida Keys, USA. We collected Rsw time series data for 4 years from the Middle Keys with about weekly resolution, as well as several 24-hr time series from coral reef sites throughout the Caribbean, to quantify the magnitude and mode of Rsw variability and to assess its likely impact on coral Sr/Ca-based paleoclimate reconstructions. Continuous seawater samples were collected with four bottom-mounted osmotic pumps placed on a transect from inside Florida Bay to approximately 10 km offshore. In Florida Bay, Rsw displays annual cycles with high values in the summer and an amplitude of 0.4–0.5 mmol/mol that attenuates and becomes less periodic with increasing distance from the coast. We propose that this seawater Sr/Ca cycle may be due to a seasonal biogeochemical process related to the precipitation and dissolution of CaCO3 in Florida Bay, with the signal carried onto the shelf through mixing. The annual Rsw cycle can explain the low Sr/Ca–SST slopes that have been reported in local corals. Pseudo-coral Sr/Ca calculations indicate that Rsw causes enhanced uncertainty in reconstructed SST, even at the site 10 km offshore. The 24-hr time series show smaller diurnal variability in seawater Sr/Ca that appears to reflect local tides and hydrography, suggesting a similar potential for aberrant coral Sr/Ca–SST calibrations at other reef locations.